Paintball mines are good, too—they explode and splatter paint on the perp.
You can buy them or DIY.
http://www.hustlepaintball.com/mines
http://www.instructables.com/id/Build-a-Paintball-Mine/
I see real uses for those.
Sure they would destroy the sign but they would seriously cut down on repeat thefts.
"Ceterum censeo Hillary esse delendam."
Garde la Foi, mes amis! Nous nous sommes les sauveurs de la République! Maintenant et Toujours!
(Keep the Faith, my friends! We are the saviors of the Republic! Now and Forever!)
LonePalm, le Républicain du verre cassé (The Broken Glass Republican)
#19 for later